A secondary battery such as a lithium ion secondary battery has an electrode plate group formed by superposing positive electrode plates and negative electrode plates alternately such that separators are interposed between the positive electrode plates and the negative electrode plates. As one of devices for producing such an electrode plate group, a zigzag stack type production device is available in which a continuous body of the separator is folded zigzag, the positive electrode plates and the negative electrode plates are inserted into the respective furrows of the zigzag-folded separator, and the resulting composite is crushed flat (see, for example, Patent Document 1).